CMP0052: Do not warn when include dir is not in source or build tree

When the policy was added by commit 783bce29 (Export: Disallow exported
interface includes in src/build tree, 2014-03-31) it accidentally left a
code path that would warn when the include dir is in the install tree
but *not* in the source or build tree.  Fix that.
Merge topic 'fix-CMP0052' into next

9e0b3153 CMP0052: Do not warn when include dir is not in source or build tree